isl_scheduler.c: copy_edges: extract out graph_edge_tables_add

isl_scheduler.c: copy_edges: extract out graph_edge_tables_add

This function will be reused in the next commit.

Note that the original code checked the edge tables of the source graph
to determine whether an edge is of a given type, while the new code
checks the types of the edge directly.
This allows graph_edge_tables_add to be independent of the source graph.
For edge types that may appear multiple times between the same pair
of nodes, this means that all edges will be added to the edge table,
but each will overwrite the previous one.
In the end, the edge referenced from the edge table will be
the last such edge, which is also the one that is referenced
from the edge table in the source graph.

Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>